May is the month when the European pine sawfly larvae feed on the old needles of pine trees. Larvae are usually present from the beginning of the month, or about the time redbuds begin to bloom. Clusters of the gray-green larvae often go undetected until the middle or end of the month when larvae are larger and damage is more advanced. Attack is most often observed on mugo, Scots and red pine though attack on other species is possible.
Defoliation by European pine sawfly is not fatal because only the old needles will be consumed. New growth is protected by a feeding deterrent and clusters of larvae, upon reaching the emerging new growth, turn and move to another branch. Severe feeding may stress newly transplanted or other struggling trees, and heavy defoliation can seriously detract from the overall beauty of the tree or shrub.
Spraying for sawfly larval defoliation is advised when the number of colonies exceeds what can be shaken or picked from the trees by hand. Large trees and Christmas tree plantations are an example. Early detection of sawfly larval colonies is an advantage because horticultural oil sprays can be used against small larvae with good results.
Horticultural oil is the newer term for what we formerly called "dormant oil." The new oil products are more carefully refined and can be used during the growing season as well as during the plant's dormant stage. Follow label directions very carefully -- summer applications of oil are at a lower rate than dormant applications.
Chemical control of larger larvae, when warranted, requires using Sevin or Orthene. Read and follow label directions. A single application is usually sufficient.
This article originally appeared in the May 19, 1993 issue, p. 73.